Compostable multilayer structures, methods for manufacture, and articles prepared therefrom

ABSTRACT

A compostable multilayer structure in the form of a film, sheet, laminate, and the like, which comprises a core layer having a first surface and a second surface, a first blocking reducing layer covering the first surface of the core layer, and a second blocking reducing layer covering the second surface of the core layer. The core layer contains a lactic acid residue-containing polymer and has a glass transition temperature (T g ) below about 20° C. The first and second blocking reducing layers include a polymer composition of a hydrolyzable polymer and have a T g  above about 50° C. The multilayer structure can be used for preparing bags and wrappers.

FIELD OF THE INVENTION

The present invention relates to compostable multilayer structures, methods for the manufacture of compostable multilayer structures, and articles prepared from compostable multilayer structures. More particularly, the compostable multilayer structures have desirable properties of flexibility and tear resistance and can provide useful disposable bags or wrappers.

BACKGROUND OF THE INVENTION

Plastic trash bags and wrappers are primarily made of hydrocarbon polymers such as polyethylene, polypropylene, or polyvinyl polymers. While hydrocarbon polymers can be useful for commercially manufacturing trash bags and wrappers having adequate flexibility and puncture and tear resistance, they are resistant to degradation and mineralization and have a tendency to build up in land fills. Under most conditions, hydrocarbon polymers take a long time to decompose. In addition, hydrocarbon polymers are not manufactured from renewable resources.

Hydrocarbons have been combined with starch in attempts at increasing degradability. Trash bags which incorporate starch can be physically degradable, which means that they become broken into many small parts as the starch biodegrades. The hydrocarbon component, however, remains resistant to degradation and mineralization. In certain circumstances, it is believed that the hydrocarbon component has a tendency to encapsulate the starch thereby preventing further biodegradation of the starch. Furthermore, materials incorporating large amounts of starch can be very sensitive to moisture and can have mechanical properties which vary considerably with humidity.

Attempts have been made at developing thermoplastic films having degradable properties. For example, U.S. Pat. No. 4,133,784 to Otey et al. describes degradable mulch films with improved moisture resistance prepared from starch and ethylene/acrylic acid copolymers. U.S. Pat. No. 5,091,262 to Knott et al. describes a multilayer polyethylene film containing a starch filled inner layer, and prodegradant filled outer layers. U.S. Pat. No. 5,108,807 to Tucker describes a multilayer thermoplastic film having a core layer made of polyvinyl alcohol, and outer layers made of polyethylene and prodegradant. U.S. Pat. No. 5,391,423 to Wnuk et al. describes multilayer films prepared from various biodegradable polymers for use in disposable absorbent products, such as diapers, incontinent pads, sanitary napkins, and pantyliners.

Many biodegradable polymers have been found to possess the desirable characteristics of biodegradability and compostability. At room temperature, however, many biodegradable polymers are either too brittle to provide the desired puncture and tear resistance necessary for commercially acceptable trash bags, or they do not have adequate stability for storage and transport. In addition, many biodegradable polymers are difficult to process into films using commercial manufacturing lines.

SUMMARY OF THE INVENTION

Compostable multilayer structures with desired properties of flexibility and tear resistance are provided by the present invention. The compostable multilayer structures are preferably in the form of films, sheets, laminates and the like. The compostable multilayer structures can be manufactured into disposable consumer products such as bags, wrappers, cups, and the like, which can degrade when subjected to composting conditions.

The compostable multilayer structures can be provided in various layered arrangements. A preferred compostable multilayer structure includes a core layer having a first surface and a second surface, a first blocking reducing layer covering the first surface of the core layer, and a second blocking reducing layer covering the second surface of the core layer. Preferably, the core layer has a glass transition temperature (T_(g)) below about 20° C., and the first and second blocking reducing layers have glass transition temperatures above about 50° C.

Applicants discovered that certain desirable properties of compostable polymer compositions, such as flexibility, tear resistance, and puncture resistance, can be adjusted by controlling the glass transition temperature thereof. For example, for many compostable polymer compositions, such as hydrolyzable polymer compositions, reducing the (T_(g)) provides a layer having increased flexibility, tear resistance, and puncture resistance to commercially acceptable levels for bags and wrappers. In addition, Applicants discovered that certain polymers compositions can be used to provide blocking reducing layers when applied over the compostable polymer compositions having increased flexibility, tear resistance, and puncture resistance. As used in the context of the present invention, blocking occurs when polymer composition layers fuse or stick together. The extent of blocking is evaluated relative to the degree of fusion between the layers or tackiness of the layers. Many polymer compositions having low glass transition temperature have been found to possess increased incidence of blocking. Applicants discovered, however, that resistance to blocking can be adjusted by controlling the glass transition temperatures. For example, for many compostable polymer compositions, such as certain hydrolyzable polymer compositions, an increased glass transition temperature tends to reduce blocking.

The layers of the compostable multilayer structures are preferably made of materials which are compostable, such as polymer compositions which include, for example, hydrolyzable polymers. Exemplary hydrolyzable polymers include copolymers and polymer blends of poly(lactic acid), poly(lactide), poly(glycolide), poly(hydroxy butyrate), poly(hydroxy butyrate co hydroxy valerate), poly(caprolactone), poly(ethylene-oxylate), poly(1,5-dioxepan 2-one), poly(1,4-dioxepan 2-one), poly(p-dioxanone), poly(delta-valerolactone), poly (trimethylene carbonate), and polyesters. The polyesters which are preferred include aliphatic polyesters because of their ability to hydrolyze to generally biodegradable units. It should be appreciated that lactic acid residue containing polymers such as poly(lactide) and poly(lactic acid) are preferred hydrolyzable polymers because of their composting and biodegradable properties. Even more preferred are copolymers prepared from lactide or lactic acid and epoxidized multifunctional oil, such as soybean oil or linseed oil.

The polymers which can be used to provide the layers of the multilayer structure should have a molecular weight which is sufficient to provide a polymer composition having film or sheet forming properties. This means that the molecular weight should be sufficiently high so that the polymer composition can form a sheet or film having integrity, and that the molecular weight should not be too high that the polymer composition is too viscous and has problems forming a sheet or film using commercial film or sheet forming equipment. Moreover, it should be appreciated that the molecular weights of the polymers used to provide the various layers can be different, reflecting the desired properties of the individual layers. For example, the molecular weight of the polymer used to prepare the core layer should be sufficiently high to provide sufficient tear strength and puncture resistance, and the molecular weight of the polymers used to prepare the blocking reducing layers should be sufficient to provide the desired glass transition temperature. Practically, it is believed that this generally corresponds with polymers having a number average molecular weight in the range of about 50,000 to about 200,000, and a weight average molecular weight in the range of about 100,000 to about 600,000. To provide sufficient flexibility and puncture and tear resistance, it has been found that the polymer used to prepare the core layer should have a number average molecular weight between about 80,000 and 200,000, more preferably between about 90,000 and 175,000, and even more preferably between about 100,000 and 150,000. The blocking reducing layers should have a number average molecular weight above about 50,000. As will be discussed in more detail, the selection of the molecular weight of the polymers used in the various layers can be adjusted to provide viscosity matching characteristics during, for example, coextrusion.

It is understood that the low glass transition temperature is responsible for providing the multilayer structure with desired flexibility and tear resistance. Accordingly, it is desirable to provide the glass transition temperature of the core layer below the temperatures at which the multilayer structure will be used. It has been found that for most conditions of use at room temperature, a T_(g) below about 20° C. should be acceptable. At cooler conditions, it is preferred that the core layer should have a T_(g) below about 5° C., and under more extreme conditions, a T_(g) below about -10° C. would be preferred.

A preferred technique for reducing the glass transition temperature of the core layer is to incorporate therein an effective amount of plasticizers into the polymer composition which forms the core layer. Generally, this means that the plasticizer can be included to provide a concentration level of about 10 to 35 percent by weight, and more preferably a concentration level of about 12 to 30 percent by weight. It is preferred that the plasticizer is biodegradable, non-toxic, compatible with the resin, and relatively nonvolatile. Plasticizer in the general classes of alkyl or aliphatic esters, ether, and multi-functional esters and/or ethers are preferred.

When the core layer has a glass transition temperature below the temperature of use of the multilayer structure, it has been found that the core layer suffers from blocking. It should be appreciated that blocking occurs when polymer layers fuse together. The extent of blocking is a function of the degree that the layers fuse together. Layers which are highly blocked will be almost totally fused together. Blocking is a particularly undesirable property for certain articles such as bags and wrappers which are commonly stored in a roll or other arrangement where the layers are in contact.

Applicants have found that blocking can be reduced by the incorporation of blocking reducing layers in the compostable multilayer structure. Preferably, the blocking reducing layers are hydrolyzable polymers such as lactic acid residue containing polymers having a number average molecular weight above 50,000. If desired, the blocking reducing layers can include anti-blocking agents to reduce blocking. Exemplary anti-blocking agents include poly(hydroxy butyrate co hydroxy valerate), cellulose acetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate, cellulose acetate butyrate, cellulose propionate butyrate, and mixtures thereof. The amount of anti-blocking agent should determined based upon its effect in reducing blocking.

In an alternative embodiment of the present invention, the compostable multilayer structure can be provided as a two layer structure having a core layer and a blocking reducing layer. It is believed that this compostable multilayer structure can be stored in the form of a roll so that both sides of the core layer are adjacent a blocking reducing layer.

Methods for manufacturing compostable multilayer structures are provided by the present invention. A preferred method includes the step of coextruding layers of compostable polymer compositions to form a multilayer structure. The coextrusion step can be carried out by a variety of techniques. One technique involves combining the separate melt streams either in the die or by simultaneously casting. An alternative technique involves combining or casting two or more streams onto a substrate, such as paper or other form of web. The layers of the multilayer structure preferably include a core layer having a first surface and a second surface and a T_(g) below about 20° C.; a first blocking reducing layer covering the first surface of the core layer, and having a T_(g) above about 50° C.; and a second blocking reducing layer covering the second surface of the core layer, and having a T_(g) above about 50° C.

The structure may be a film formed from an extruded melt by any of several means. The structure may be cast and quenched, either onto a drum, a belt, in water, or the like. The cast film may be subsequently oriented, either uniaxially or biaxially, using conventional equipment such as drawing on heated rollers or using a tenter-frame, or a combination thereof. The processing operation may also include crystallization (of the outer layers) and/or heat-setting of the film. The biaxially oriented film can also be subjected to additional drawing of the film in the machine direction, in a process known as tensilizing.

The film may also be processed in a blown-film apparatus, in order to achieve direct biaxial orientation directly from the melt or in a double-bubble process. The blown-film process is known in the art, as is the double-bubble process. In the blown film process the annular tube is inflated as it leaves the extruder and is cooled with an air ring, prior to collapsing and winding. The double-bubble process first quenches the tube, it is then reheated and oriented by inflating at a temperature above the T_(g) but below the crystalline melting point (if the polymer is crystalline).

For lactic acid residue containing polymers based outer layers, the preferred temperature for orienting in the double bubble process is believed above the T_(g) but below 90° C., and preferably below 75° C. If orientation occurs at a temperature which is too high, it may be difficult to obtain sufficient crystallinity, and the resulting polymer composition will not be resistant to blocking at temperatures greater than 50° C.

The film may also be subjected to rolling, calendaring, coating, embossing, printing, or any of the other typical finishing operations known in the art.

The core layer, with a T_(g) less than 20° C. and (typically) amorphous, can be difficult to handle in pelletized form. A preferred method for preparing the core layer is to start with a polymer which has a T_(g) greater than about 40° C. and inject the plasticizer into the extruder during the film forming process. This "on-line" plasticization is advantageous because it removes the need to pelletize and handle the low T_(g) material.

Compostable multilayer sheets are provided by the present invention. The sheet has a thickness greater then 10 mils (0.010 inch). The sheet can be used as thermoformed rigid container, cups, tubs, dinnerware, etc. In most applications, it is understood that the sheet will have a thickness less than 150 mils.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a multilayer structure in the form of a film according to the principles of the present invention;

FIG. 2 is a cross-sectional view of an alternative embodiment of a multilayer structure in the form of a laminate having a paper substrate according to the principles of the present invention; and

FIG. 3 is a graph comparing the rate of biodegradation of the multilayer film of Example 2, kraft paper, and cellulose.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a multilayer structure having compostable properties. This means that the multilayer structure will break down and become part of a compost upon being subjected to physical, chemical, thermal, and/or biological degradation in a solid waste composting or biogasification facility. As used in this application, a composting or biogasification facility has a specific environment which induces rapid or accelerated degradation. Generally, conditions which provide rapid or accelerated degradation, compared with storage or use conditions, are referred to herein as composting conditions. In the context of the present invention, the multilayer structure may be referred to as a compostable multilayer structure.

In order to provide a compostable multilayer structure, the components of the multilayer structure should be compostable and biodegradable during composting/biogasification, or in compost amended soil, at a rate and/or extent comparable to that of known reference materials such as cellulose or paper. Basically, this means that the components should be degradable within a time frame in which products made therefrom, after use, can be recycled by composting and used as compost. It should be understood that certain materials such as hydrocarbons and other polymeric resins including polyethylenes, polypropylenes, polyvinyls, polystyrenes, polyvinyl chloride resins, urea formaldehyde resins, polyethylene terephthalate resins, polybutylene terephthalate resins, and the like are not considered compostable or biodegradable for purposes of this invention because they take too long to degrade when left alone in a composting environment.

It is preferred that as large a percentage as possible of the materials making up the multilayer structure should be compostable and biodegradable. Preferably, the materials can be chemically or biologically broken down, then mineralized by microorganisms in a biologically active environment to simple molecules, such as, carbon dioxide, methane, and water, leaving biomass and naturally occurring assimilation products. In assessing biodegradability, both the rate of biodegradation and the ultimate extent of biodegradation are important considerations. It is preferred that the rate of biodegradation and the extent of biodegradation of the materials used in the multilayer structure should be comparable to known reference materials such as cellulose or paper.

The rate and extent of biodegradation of the multilayer structure can be correlated to known biodegradable materials, such as kraft paper or cellulose, using ASTM D5338-92 Test Method for Determining Aerobic Biodegradation of Plastic Materials Under Controlled Composting Conditions. This is a laboratory test which compares the rate of biodegradation of a test sample to that of a known biodegradable material by determining the amount of CO₂ evolved from the compost with and without the test sample. A modified version of the ASTM D5338-92 test can be used to more conveniently approximate large scale composting conditions. This modified test is referred to as the first modified test and is performed according to ASTM D5338-92 except that a constant temperature of 58° C. is provided. The amount of material biodegraded is calculated based upon measuring the amount of carbon dioxide evolved therefrom. A second modified version of ASTM D5338-92 can be used to determine the degradation at soil conditions. The second modified test is carried out according to ASTM D5338-92 except that a temperature of 30° C. is used and the media is soil at approximately 70% of its moisture holding content (ASTM D 425).

Test results for biodegradation according to the first modified ASTM D5338-92 test are provided in FIG. 3 where the cumulative percent biodegradation (referred to as the biodegradation value) is measured as a function of time for the multilayer structure prepared in Example 2, kraft paper, and cellulose. The details of this test are described in Example 9. For the results plotted in FIG. 3, a biodegradation value of 70 percent at 40 days means that at least 70 percent of the carbon in the multilayer structure has been converted to carbon dioxide and microbial biomass after composting under conditions of the first modified test for 40 days. For most multilayer structures of the present invention, it is preferred that they have a biodegradation value of at least 50 percent after 40 days, and even more preferably at least 60 percent after 40 days. In addition, it is preferred that they possess a biodegradation value of at least about 70 percent after 60 days. For slower materials, biodegradation values of at least about 20 percent after 40 days and/or at least about 30 percent after 60 days can be provided.

Another way of characterizing the rate and extent of biodegradation of the multilayer structure of the invention is to compare it with the rate and extent of biodegradation of known compostable and biodegradable materials such as kraft paper and cellulose. Generally, it is desirable that the multilayer structure will have a biodegradation value which is at least about 50 percent, and more preferably at least about 60 percent, of the biodegradation value of kraft paper or cellulose after 40 days in a standard compost as provided in ASTM D5338-92.

In a preferred embodiment of the invention, the entire multilayer structure will eventually decompose under composting conditions. It should be appreciated, however, that practical composting conditions may terminate before the active stage of composting is complete. As a result, multilayer structures of the invention can then be placed in soil or otherwise disposed of before complete decomposition occurs. It is believed degradation of a multilayer structure which is not completely degraded after composting will continue at a slower rate when placed into soil.

It should be understood that the multilayer structure of the invention can include materials which are not compostable or biodegradable under short period composting conditions. Such materials can be incorporated to provide desired physical properties, such as barrier properties, and should be kept to a minimum since it is desirable to provide a multilayer structure having a high degree of degradability under composting conditions. Exemplary types of materials include thermoplastic resins, such as polyethylene, polypropylene, polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyvinyl alcohol, poly(acrylonitrile), and select highly substituted cellulose esters.

According to Example 9, the multilayer structure of Example 2 has a biodegradation value of about 90 percent after 60 days according to the modified ASTM D5338 test, conducted at a constant temperature of 58° C. It should be appreciated that the extent of crystallization in the lactic acid residue containing polymers can alter this value. It is believed that a more crystalline lactic acid residue containing polymer will generally biodegrade more slowly than a less crystalline lactic acid residue containing polymer.

The multilayer structure of the invention can be provided as films, sheets, laminates, and the like. Films can be used in applications such as disposable bags, wrappers, personal hygiene products, packaging materials, agricultural mulch films, and the like. Exemplary disposable bags include trash bags, sandwich or snack bags, grocery bags, waste bin liners, compost bags, food packaging bags and the like. Exemplary disposable wrappers include food wrappers such as fast food wrappers, food packaging films, blister pack wrappers, skin packaging and the like. Sheets can be used in applications including thermoformed rigid containers, cups, tubes, dinnerware, cup lids, deli trays and the like. Laminates include coated paper which can be used, for example, as boxes, multiwall bags, multiwall containers, spiral wound tubes (e.g., mailing tubes), and the like. In situations where the multilayer structure is in the form of a film or sheet, it may be desirable to ensure that the film or sheet possess the properties of tear resistance, quietness, and impact resistance. Transparent structures may also be of benefit for any packaging application, where it is desirable to see the package contents. The multilayer structure of the invention can also be prepared using coextrusion blow molding, to directly produce articles such as rigid containers, tube, bottles, and the like. For certain applications, such as use in a compostable lawn refuse bag, it may be desirable to have the multilayer structure substantially transparent to visible light. This allows rapid determination of the contents before shipping to a compost facility, or identification of contents under a wrapper.

Applicants have found that many biodegradable polymers do not possess desirable physical properties for use as single layer films or sheets because they have high glass transition temperatures, structures which do not rapidly crystallize, low melting points, or are difficult to process on conventional machines. A particular problem is often encountered when trying to process biodegradable polymers in conventional process equipment. In contrast, it is believed that certain hydrocarbons can be very useful for forming single layer bags because they have very low glass transition temperatures, crystallize quickly, have relatively high melting temperatures, and are easy to process. Polyethylene, for example, has a T_(g) of -100° C. and a T_(m) of over 100° C. which makes it ideally suited for use in producing single layer trash bags. As will be discussed in more detail, it is believed that viscosity modified polymers having branching and bridging structures as taught in U.S. Pat. No. 5,594,095 issued to Gruber et al. on Jan. 14, 1997, and incorporated herein by reference, can provide desirable melt processing and viscosity characteristics although glass transition temperature can remain high.

The glass transition temperature of a polymer is considered too high for use as a flexible film or flexible sheet if it is above the temperature at which the polymer will be used for a given application, which can be referred to herein as the ambient temperature (T_(a)). When the polymer is used at a T_(a) which is lower than the T_(g) of the polymer, the polymer will typically be too brittle which can result in cracking or fracturing of a layer formed therefrom. An example of a polymer having a T_(g) which is too high is linear or viscosity modified lactic acid residue containing polymer which has a T_(g) of about 54° C. It is presently understood that branching or bridging does not significantly effect glass transition temperature.

It is understood that crystallinity is an important characteristic of a polymer which can be used to reduce blocking between layers of the polymer compositions. As discussed above, blocking occurs when layers of polymer compositions fuse together. It is a particularly undesirable property when it is exhibited by trash bags because it causes the sides of the bag to stick together, thereby preventing the bag from opening. It is believed that the rate and extent of crystallization of a polymer effects blocking both in the final product and during manufacture. For example, it is understood that if the polymer crystallizes sufficiently quickly, it is believed that the tendency to block can be reduced. On the other hand, polymers which crystallize slowly will have a tendency to block in the process equipment when recently formed films or sheets fuse together. An example of a polymer which does not crystallize sufficiently quickly under processing conditions is poly(caprolactone). It is believed, however, that for many polymers such as poly(caprolactone), processing conditions can be modified to reduce blocking. For example, it is believed that the double bubble blown film process can reduce blocking in poly(caprolactone) polymer compositions.

Some biodegradable polymers are not suitable for single layer bags because they have a melting point (T_(m)) which is too low. A low T_(m) renders a polymer difficult to process, and requires cooling below its T_(m) to induce crystallization. Several aliphatic polyesters have a T_(m) which is too low. Also, if the storage or use temperature exceeds T_(m) then the film will tend to fuse and lose integrity. An exemplary aliphatic polyester, such as polycaprolactone, requires a crystallization temperatures of room temperature or below which is difficult to achieve in most blown film or cast film facilities. Exemplary aliphatic polyesters having desirable T_(m) include polyglycolide, polylactide, and poly(hydroxy butyrate). Unfortunately, these polymers have a T_(g) which is too high.

Applicants have found ways to provide biodegradable polymer compositions having glass transition temperatures lower than ambient temperature. Various methods within the scope of the invention include providing blends of polymers or other additives, using copolymers, incorporating a plasticizer, and the like. These methods are discussed in more detail below. Although the resulting biodegradable polymer compositions have a glass transition temperature lower than ambient temperature, it has been observed that they can suffer from blocking when formed into a film or sheet. In order to overcome the blocking problem, Applicants discovered that certain biodegradable polymer compositions, such as amorphous or semi-crystalline polylactide polymer compositions, can be formed into thin layers and used as blocking reducing layers.

The Compostable Multilayer Structure

Now referring to FIG. 1, a preferred embodiment of the multilayer structure according to the present invention is shown at reference numeral 10 in the form of a film. The multilayer film 10 includes a core layer 12, a first blocking reducing layer 14, and a second blocking reducing layer 16. The first blocking reducing layer 14 covers the first surface 13 of the core layer 12, and the second blocking reducing layer 16 covers the second surface 15 of the core layer 12. In the arrangement shown in FIG. 1, the core layer 12 is in contact with both the first blocking reducing layer 14 and the second blocking reducing layer 16. It should be understood, however, that for one layer to "cover" another layer, it is not necessary that the layers be in contact with each other. It should be appreciated that another layer or material can be placed therebetween. For example, a layer of adhesive, polymer, foil, or other material, such as paper, can be placed between the core layer and the blocking reducing layer. Various properties, such as, vapor resistance, chemical resistance, adhesion, tensile strength, and the like, can be provided by selecting layers in addition to those shown in the multilayer film 10.

It should be appreciated that the multilayer film can be provided without a second blocking reducing layer. The film could be stored in roll form so that the core layer contacts both sides of the blocking reducing layer. The film could then be unrolled prior to use, for example, as a wrapper or covering.

In an alternative embodiment of the invention shown in FIG. 2, a laminated paper product 20 is provided. The laminated paper product has a paper substrate 21, a core layer 22, and a blocking reducing layer 24 covering the core layer 22. It should be appreciated in this embodiment that the paper substrate 21 can also function as a blocking reducing layer.

The relative thickness of the core layer and the blocking reducing layers can be determined by taking a cross section and examining by optical microscopy. A second method would be to take a cross section and examine with an FTIR probe to determine composition across the profile. A third way is by measuring the flow rate of polymer streams into the multilayer die and calculating the resulting thicknesses. It is generally desirable for the blocking reducing layers to be as thin as possible to provide sufficient resistance to blocking and sufficient coverage over the core layer. In most structures, including films, sheets and laminates it is believed that the blocking reducing layers will be considered films, and may herein be referred to as films in the context of the present invention. It is understood that the core layer, generally, is primarily responsible for providing flexibility and tear and puncture resistance. Accordingly, it is usually preferred to maximize the core layer relative to the blocking reducing layers.

It should be appreciated that it would be desirable to recycle or use scrap or regrind recovered during the production of a multilayer structure in another multilayer structure. In some applications of the multilayer structure, such as thermoform operations, it is common for fairly large amounts of scrap or regrind to be generated. It is often preferred that the scrap or regrind can be remelted and used in the production of the core layer of another multilayer structure in order to keep the economics favorable. In this context, it can be desirable for the scrap or regrind to lose it multilayer properties upon remelting. Accordingly, maintaining the blocking reducing layers as a small percentage of the overall composition will allow the scrap or regrind to be added back into the core layer composition with only minor effects on glass transition temperature.

For many applications where the blocking reducing layer is extruded, the thickness of the blocking reducing layer should be sufficient to provide a continuous layer and/or desired blocking resistance. It is believed that this usually corresponds with a lower limit of at least about 0.05 mil. If the thickness of the blocking reducing layer is much less than 0.05 mil, it has been found to be difficult to maintain a continuous coating. In most applications, it is believed that the thickness of the blocking reducing layers should be less than 0.5 mil, more preferably less than 0.3 mil, and even more preferably less than 0.1 mil. The core layer can be essentially any size so long as it provides the desired properties.

For most multilayer films, such as the one depicted in FIG. 1, it is believed that the total thickness of the film will usually be less than about 10 mil, and more preferably between about 1 mil and about 3 mils. Since it is desirable to keep the ratio of thicknesses of a blocking reducing layer to the overall thickness of the film as low as possible, this ratio should be between about 5% and 25%. The ratio of the combined thickness of the blocking reducing layers to the overall thickness of the multilayer structure should be less than about 40%, and more preferably less than about 30%. Accordingly, at least about 70% of the thickness of the multilayer structure should be core layer.

It is believed that a multilayer sheet will have a thickness of at least about 10 mil or greater. Multilayer layer structures which include a paper layer or substrate can have a thickness, exclusive of paper substrate, of 0.5-3 mil. Generally, the thickness of the blocking reducing layers will have essentially the same values described above so long as they provide sufficient blocking resistance.

The Materials Of The Multilayer Structure

The polymeric material which can be used in the multilayer structure is preferably a degradable polymer. A preferred type of degradable polymer is hydrolyzable polymers which can be characterized as being hydrolytically degradable. This means that chemical bonds in the molecule are subject to hydrolysis, thus producing smaller molecules. More preferably, the "hydrolyzable polymers" should additionally be "biodegradable" in a biologically active environment to simple molecules. It is preferred that the hydrolyzable polymers can be degraded by water at neutral pH, with cleavage of polymer linkages. Lactic acid residue containing polymers such as poly(lactic acid) polymers and polylactide polymers are preferred hydrolyzable polymers because they generally hydrolyze to lactic acid. The hydrolysis of lactic acid residue containing polymers can be manifested by a decrease in molecular weight as the molecules break down, and by a "weight loss" as water soluble lactic acid is formed.

The hydrolyzable properties of polymers can be evaluated by placing a sample of the polymer in an aqueous bath buffered to a pH of 7.4 with a phosphate buffer and maintained at a temperature of 75° C. For the purposes of this application, if the polymer is hydrolyzable, it will show significant drop in molecular weight within 3 days, and some weight loss within 14 days. For polymers which are water insoluble, the percent of the polymer considered hydrolyzable is the percent of initially insoluble material which is lost from the sample in 50 days. By this definition, many lactic acid residue containing polymers are 100% hydrolyzable. The crystallinity of a polymer, however, can effect its hydrolyzable properties. More crystalline polymers generally take longer to hydrolyze.

The hydrolyzability test for samples containing water soluble polymers would be modified to recover the solubilized material and determine its molecular weight. The fraction of material with molecular weight equal to or less than the expected monomeric degradation product would be considered to have been hydrolyzed.

Exemplary types of hydrolyzable polymers include poly(lactide), poly(lactic acid), poly(glycolide), poly(hydroxy butyrate), poly(hydroxy butyrate co hydroxy valerate), poly(caprolactone), poly(ethylene-oxylate), poly(1,5-dioxepan 2-one), poly(1,4-dioxepan 2-one), poly(p-dioxanone), poly(delta-valerolactone), poly (trimethylene carbonate), and polyester. The polymers can be copolymers and polymer blends. Preferably, the polyesters are aliphatic polyesters because of their hydrolysis to generally biodegradable units.

Lactic acid residue containing polymers are particularly preferred in the present invention due to their hydrolyzable and biodegradable nature. One theory of the degradation of lactic acid residue containing polymers is that they can be degraded by hydrolysis at hydrolyzable groups to lactic acid molecules which are subject to enzymatic decomposition by a wide variety of microorganisms. It should be appreciated, however, that the precise mechanism of degradation is not a critical feature of the present invention. Rather, it is sufficient that one recognizes that polymers which provide similarly rapid degradation to naturally occurring end products can be useful in the present invention. U.S. Pat. No. 5,142,023 issued to Gruber et al. on Aug. 25, 1992, the disclosure of which is hereby incorporated by reference, discloses, generally, a continuous process for the manufacture of lactide polymers from lactic acid. Related processes for generating purified lactide and creating polymers therefrom are disclosed in U.S. Pat. Nos. 5,247,058; 5,247,059; and 5,274,073 issued to Gruber et al., the disclosures of which are hereby incorporated by reference. It should be appreciated that selected polymers from these patents having the physical properties suitable for use in the present invention can be utilized. Generally, polymers according to U.S. Pat. No. 5,338,822 issued to Gruber et al. on Aug. 16, 1994 and U.S. Pat. No. 5,594,095 issued to Gruber et al. on Jan. 14, 1997, which are incorporated by reference, can be used in the present invention. Exemplary lactic acid residue containing polymers which can be used are described in U.S. Pat. Nos. 5,142,023; 5,274,059; 5,274,073; 5,258,488; 5,357,035; 5,338,822; 5,359,026; 5,484,881; 5,536,807; and 5,594,095, the disclosures of which are incorporated herein by reference. Polylactide polymers which can be used in the invention are available under the tradename EcoPLA™.

By know it should be appreciated that the term lactic acid residue containing polymer includes polymers containing about 50%, by weight, or more lactic acid residue units which, under certain conditions, will hydrolyze to lactic acid or derivative thereof. The remaining components of the lactic acid residue containing polymers can include non-lactic acid residues. Preferably, the lactic acid residue containing polymer is least about 70%, and more preferably at least about 90%, lactic acid residue. In a preferred embodiment, the lactic acid residue containing polymer contains less than about 2%, by weight, non-lactic acid residue.

Lactic acid residue containing polymers are generally prepared from monomers which include lactic acid, lactide, or combination thereof. It should be understood that other structural units which, when polymerized, have a structure similar to polymerized lactic acid or lactide can be used. Rather than focusing on how the lactic acid residue containing polymers are prepared, it should be understood that what is important is that the lactic acid residue containing polymers have characteristics which render them susceptible to hydrolysis and thereby enhance degradability or biodegradability. It is these characteristics which are important rather than the strict chemical composition of the polymer. However, polymers which are considered lactic acid residue containing polymers include poly(lactide) polymers, poly(lactic acid) polymers, and copolymers such as random and/or block copolymers of lactide and/or lactic acid. Lactic acid components which can be used to form the lactic acid residue containing polymers include L-lactic acid and D-lactic acid. Lactide components which can be used to form the lactic acid residue containing polymers include L-lactide, D-lactide, and meso-lactide.

A particularly preferred type of polylactide polymer includes viscosity modified polylactide which is described in detail in U.S. Pat. No. 5,359,026 and U.S. Pat. No. 5,594,095 issued to Gruber et al. on Jan. 14, 1997, entitled "Viscosity-Modified Lactide Polymer Composition And Process For Manufacture Thereof," the application being incorporated by reference. Viscosity modified polylactide polymers are important because they provide desirable processing characteristics such as reduced viscosity, increased melt strength, and hence improved bubble stability.

The viscosity modified polylactide polymers which can be used in the present invention have increased molecular interaction among the polymer chains. The increased molecular interaction being provided by, for example, providing bridging in the polylactide polymer, providing branching with the polylactide polymer, and increasing the weight average molecular weight of the polylactide. The "bridging" refers to providing bonding between long polymer polylactide-based chains. This can be accomplished by using free radical generating peroxides to cleave substituents from the polylactide backbones thereby generating polymer radicals that can bond with other polymer radicals, or by reaction of multifunctional chain extenders to link polymer chains together. The "branching" refers to providing pendent groups from linear polylactide-based polymer chains or providing long polymer segments joined to one another through a residue. This can be accomplished by introducing an initiator into the lactide reactants, by using a non-initiating agent such as an epoxidized hydrocarbon or an epoxidized oil, or by copolymerizing with molecules containing at least two cyclic ester rings.

Particularly preferred viscosity modified polylactide polymers include copolymers of lactide and epoxidized multifunctional oil such as epoxidized linseed oil and epoxidized soybean oil. In many situations, it is preferred that the polymer is prepared from 0.1 to 0.5 weight percent epoxidized multifunctional oil and molten lactide monomer. Catalyst can be added, and the mixture can be polymerized between about 160° C. and 200° C. The resulting polymer preferably has a number average molecular weight of about 80,000 to about 140,000.

It should be appreciated that lactic acid residue containing polymers are sensitive to high temperatures which creates processing and storage life problems if not adequately addressed. U.S. Pat. No. 5,338,822 describes how lactic acid residue containing polymer stability can be provided during melt processing.

As discussed above, many biodegradable polymers, such as linear lactic acid residue containing polymers, are generally too crystalline and brittle for use as a flexible film and/or sheet. Their T_(g) is generally above 50° C., and it has been observed that they provide a film or sheet having low impact resistance and tear resistance. Tear resistance of a typical polylactide film having a T_(g) above 50° C. is less than about 6 gm/mil. These physical properties render films or sheets prepared therefrom poor candidates for use as bags or wrappers. Articles such as trash bags, grocery bags, food wrappings, and the like should be flexible and resistant to tearing and puncturing.

Applicants discovered that by lowering the glass transition temperature (T_(g)) of biodegradable polymers to about 20° C. or less, it is possible to provide a film or sheet having improved flexibility and tear and puncture resistance. More preferably, it is desirable to lower the T_(g) to below about 5° C., and more preferably below about minus 10° C. These glass transition temperature should be below the temperature at which the polymer is used. When the biodegradable polymer is a lactic acid residue containing polymer, a preferred method for lowering the glass transition temperature (T_(g)) is by adding plasticizer thereto. As demonstrated in Example 1, plasticizer can be added to a polylactide polymer to lower the glass transition temperature (T_(g)) from 60° C., without plasticizer, to 19° C. at a level of 20 percent, by weight, plasticizer.

For most lactic acid residue containing polymers, it is believed that the glass transition temperature can be lowered to desirable levels by adding a plasticizer component to provide a concentration of about 1 to 40 percent by weight plasticizer, based on the weight of the polymer composition. Generally, a sufficient amount of plasticizer should be incorporated to provide a desired reduction in T_(g) and increase flexibility and tear strength. It is believed that the plasticizer level should be above at least 8 percent by weight, and more preferably above at least 10 percent by weight, to provide sufficient flexibility and tear resistance. The upper limit on plasticizer can be controlled by other considerations such as loss of film or sheet integrity if too much plasticizer is used. Accordingly, the plasticizer should be included to provide a concentration level of about 10 to 35 percent by weight, and more preferably a concentration level of about 12 to 30 percent by weight.

The selection of the plasticizer can involve consideration of several criteria. Since it is generally desirable to provide as much biodegradability as possible, it is preferred to use a plasticizer which is biodegradable, non-toxic, compatible with the resin, and relatively nonvolatile. Plasticizer in the general classes of alkyl or aliphatic esters, ether, and multi-functional esters and/or ethers are preferred. These include alkyl phosphate esters, dialkylether diesters, tricarboxylic esters, epoxidized oils and esters, polyesters, polyglycol diesters, alkyl alkylether diesters, aliphatic diesters, alkylether monoesters, citrate esters, dicarboxylic esters, vegetable oils and their derivatives, and esters of glycerine. Preferred plasticizer are tricarboxylic esters, citrate esters, esters of glycerine and dicarboxylic esters. More preferably, citrate esters are preferred since it is believed that these esters are biodegradable. These plasticizer can be obtained under the names Citroflex A-4® (acetyl tri-n-butyl citrate), Citroflex A-2® (acetyl triethyl citrate), Citroflex C-2® (triethyl citrate), Citroflex C-4® (tri-n-butyl citrate) (from Morflex).

It should be appreciated that plasticizer containing aromatic functionality or halogens are less preferred because of their possible negative impact on the environment. For example, appropriate non-toxic character is exhibited by triethyl citrate, acetyltriethyl citrate, tri-n-butyl citrate, acetyltri-n-butyl citrate, acetyltri-n-hexyl citrate, n-butyltri-n-hexyl citrate and acetyltriethyl citrate, tri-n-butyl citrate, diisobutyl adipate, diethylene glycol dibenzoate, and dipropylene glycol dibenzoate. Appropriate compatibility is exhibited by acetyltri-n-butyl citrate, acetyltriethyl citrate, tri-n-butyl citrate, diisobutyl adipate, diethylene glycol dibenzoate, and dipropylene glycol dibenzoate. Other compatible plasticizers include any plasticizer or combination of plasticizer which can be blended with lactic acid residue containing polymer and are either miscible therewith or which form a mechanically stable blend.

Volatility is determined by the vapor pressure of the plasticizer. An appropriate plasticizer should be sufficiently non-volatile such that the plasticizer stays substantially in the composition throughout the process needed to produce the multilayer structure, and to provide desired properties when the structure is used. In addition, excessive volatility can lead to fouling of process equipment. Preferred plasticizer should have a vapor pressure of less than about 10 mm Hg at 170° C., more preferred plasticizer should have a vapor pressure of less than 10 mm Hg at 200° C.

Internal plasticizer, which are bonded to the lactic acid residue containing polymer, may also be useful in the present invention. Exemplary plasticizer which can be bonded to the polymer include epoxides.

Although plasticized lactic acid residue containing polymers can provide desired tear strength, they have shown severe blocking which makes them unsuitable as single layer bags or wrappers. It should be understood that "blocking" is meant to describe the tendency of one layer of a structure to entangle, enmesh or stick to another layer. Thus, two layers exhibiting high blocking are not easily separated to form, for example, a bag. Applicants have found that while reducing the T_(g) of lactic acid residue containing polymers enhances flexibility and tear strength, it also increases or promotes blocking. This feature is demonstrated by the data in Table 1 in Example 1.

Applicants have tried several methods to reduce the blocking of a plasticized lactic acid residue containing polymer film. One method which reduced the blocking tendency was to crystallize the lactic acid residue containing polymer film. Unfortunately, the crystallized film lost desirable properties, such as tear resistance, making this method unsatisfactory.

Applicants also tried compounding with conventional organic or inorganic fillers, such as diatomaceous earth. When used with a plasticized polylactide composition, this technique produced results which initially appeared favorable and produced a film with good bubble stability and no initial blocking. However, after a short storage period the films began to fuse.

Applicants found that multilayer structures could be created which were relatively resistant to blocking and which retained desirable properties, such as elongation and tear resistance, of a plasticized lactic acid residue containing polymer composition. The blocking was reduced by using blocking reducing layers covering the core layer of plasticized lactic acid residue containing polymer. The blocking reducing layers could have a variety of compositions, provided that they reduce blocking.

Four preferred types of compositions for forming the blocking reducing layers are described below. A preferred first composition for preparing the blocking reducing layer includes amorphous lactic acid residue containing polymer having a T_(g) above 50° C. It is believed that the high glass transition temperature of the amorphous lactic acid residue containing polymer is responsible for reducing or preventing blocking. Thus, blocking can be reduced provided that the ambient or use temperature is below the T_(g) of the blocking reducing layer. It is believed that at temperatures below the T_(g) of the polymer, the molecules in the polymer are not sufficiently mobile to cause blocking.

A preferred second composition which can be used for preparing the blocking reducing layer includes semicrystalline lactic acid residue containing polymer. A semicrystalline lactic acid residue containing polymer will generally have an optical purity of greater than 85% either R or S lactic acid residues, although the overall composition can be less optically pure if the polymer is a block copolymer, rather than random. The semicrystalline lactic acid residue containing polymer provides blocking resistance to higher temperatures than the amorphous lactic acid residue containing polymer, with no blocking observed even at temperature of 90° C.

A preferred third composition which can be used for preparing the blocking reducing layer includes lactic acid residue containing polymer and a high glass transition temperature polymeric additive for reducing blocking. Preferred high T_(g) polymeric additives include polymers with a T_(g) greater than about 50° C., and more preferably greater than about 90° C. The most preferred high T_(g) polymeric additives are biodegradable and derived from renewable resources. Exemplary preferred high T_(g) polymeric additives include cellulose acetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate, cellulose acetate butyrate, cellulose propionate butyrate, terpene resins and rosin and rosin esters derived from tree sap.

A preferred fourth composition which can be used for preparing the blocking reducing layer includes a lactic acid residue containing polymer and a semicrystalline polymeric additive. Preferred semicrystalline polymeric additives will have a melting point above 90° C. and more preferably above 120° C. The most preferred semicrystalline polymeric additives are biodegradable and derived from renewable resources. Preferred semicrystalline polymeric additives include aliphatic polyester with melting points above 90° C. Exemplary preferred semicrystalline polymeric additives include poly(hydroxy butyrate) and poly(hydroxy butyrate-co-hydroxy valerate). It is believed that poly(glycolide), poly(lactide), or the stereocomplex of poly(L-lactide) and poly(D-lactide) might also be suitable for use as anti-blocking agents.

Examples of polyhydroxybutyrate (PHB) polymers and polyhydroxy butyrate/valerate copolymers (PHBV) are described in U.S. Pat. No. 4,393,167, Holmes et al., issued Jul. 12, 1983, and U.S. Pat. No. 4,880,592, Martini et al., issued Nov. 14, 1989, and U.S. Pat. No. 5,391,423, Wnuk et al., issued Feb. 21, 1995, these patents being incorporated herein by reference. Polyhydroxy butyrate/valerate copolymers are commercial available from Ceneca Corp. under the tradename Biopol. The Biopol polymers are produced from the fermentation of sugar by the bacterium Alcilagenes eutrophus. PHBV polymers are currently produced with valerate contents ranging from about 5 to about 25 mole percent. Increasing valerate content decreases the melting point, crystallinity, and stiffness of the polymer. An overview of Biopol technology is provided in Business 2000+, (Winter 1990), incorporated herein by reference.

Without being bound by theory, it is believed that a limited compatibility of the anti-blocking agent in the blocking reducing layer may be partly responsible for enhancing the anti-blocking characteristics thereof.

Applicants believe that it may be possible to modify the core layer with a blocking reducing modifier to reduce migration of plasticizer and thereby reduce blocking. An exemplary blocking reducing modifier is peroxide. Example 5 demonstrates the use of a peroxide blocking reducing modifier in a PLA and plasticizer composition. Without being bound by theory, it is believed that the blocking reducing modifier functions by modifying the core layer, possibly by reaction, so as to discourage plasticizer migration out of the core layer. It is believed that the blocking reducing modifier may bond the plasticizer to the core layer. In the case where peroxide is used as the blocking reducing modifier, it is preferably provided at a concentration of about 0.05 to about 0.5 percent by weight.

The core layer of the multilayer structure should be sufficiently flexible to be rolled or folded for packaging, to be useful for purpose intended. Preferably, the first layer should have sufficient flexibility to allow it to be folded over onto itself without cracking at the crease. It is preferred that a multilayer film according to the present invention would have a tensile modulus of less than 75,000 psi at 23° C. when tested according to ASTM D-882 method A-3.

Pigments or color agents may also be added as necessary. Examples include titanium dioxide, clays, calcium carbonate, talc, mica, silica, silicates, iron oxides and hydroxides, carbon black and magnesium oxide.

Applicants have found that the presence of residual catalysts in the lactic acid residue containing polymer structure significantly affects the stability thereof during processing. Accordingly, the catalyst level can be controlled as described in U.S. Pat. No. 5,338,822, which is incorporated herein by reference.

Forming the Multilayer Structure

The multilayer structure of the present invention can be manufactured by a variety of techniques, including coextruding blown or cast film. The preferred technique is coextrusion of blown film where all layers are extruded concurrently and are applied to each other either soon before or soon after they leave the co-extrusion die. A second preferred technique is coextrusion blow molding, for the production of articles. These techniques allow all the layers of the multilayer structure to be simultaneously extruded, stretched and combined. Other processing techniques used to prepare films, sheets, laminates and the like can be used in the present invention. Many techniques are described, for example, in Encyclopedia Of Polymer Science And Engineering, 2nd Edition, John Wiley and Sons, 1986, which is incorporated herein by reference.

As demonstrated by Example 4, it is important to match the viscosity of each layer in order to provide a structure wherein having a maximum thickness of the flexible core layer relative to the blocking reducing layers. Maximizing the core layer is believed to provide a maximum tear strength. In addition, it is believed that viscosity matching increases the compatibility between the layers. Viscosity matching can be accomplished by a variety of techniques including modifying the temperature of the die or individual polymer melt streams and/or altering the composition of the extrusion material.

When preparing the multilayer structure by blown film coextrusion or coextrusion blow molding, it is desirable to inject the plasticizer into the polymer composition which forms the core layer while it is in the extruder. This avoid having to handle pellets having a low T_(g) which would tend to stick together.

It should be appreciated how the orientation of the layers can be modified to provide desired physical properties. For example, the physical properties of a multilayer film prepared by blown film coextrusion can be altered by adjusting the blow-up-ratio (BUR) which is defined as the ratio of the diameter of the film bubble to the die diameter. For purposes of providing a bag or wrapper, it is typically desirable to provide a BUR of between about 1 and 5, and should be determined based upon the desired balance of properties in the machine direction and the tensile direction. For a balanced multilayer film, a preferred BUR of about 3:1 is appropriate. It is recognized that it may be desirable to have a "splitty" film which easily tears in one direction such as the machine direction. It is expected that a BUR of about 1:1 to 1.5:1 should provide such a multilayer structure.

As demonstrated by the data in Example 8, it is understood that migration of plasticizer from the core layer into the blocking reducing layers can be minimal over time. Without being bound by theory, Applicants hypothesize that plasticizer begins migrating into the outer/non-plasticizer containing layers, and that the presence of plasticizer allows the interfaces between the core layer and the non-plasticizer containing layers to crystallize which discourages or retards further migration of plasticizer. It is believed that by inducing crystallization at the interface, or in the boundary area in the non-plasticizer containing layers, properties including vapor and moisture diffusion can be altered.

Barrier properties, such as oxygen and water permeability, can also be affected by crystallizing and/or orienting the film. If the desired permeability cannot be achieved through processing, then a layer of additional polymer may be introduced to provide the desired barrier properties. For example, a thin layer of polyethylene, polypropylene, polyethylene terephthalate (PET), or other polymer with a water permeability of less than about 5 g-mil/100 in² -day, could be added to reduce the water permeability of a lactic acid residue containing polymer based film. It has been found that the moisture vapor transmission rate for a multilayer film, such as in Example 2, is about 25 g-mil/100 in² -day.

The oxygen permeability of lactic acid residue containing polymer based films has not been reported, but it is estimated to be about 2×10⁻¹¹ cc-cm/(cm² -sec-cm Hg). A thin layer of PVOH, EVOH, PAN, or other polymer with oxygen permeability of less than about 2×10⁻¹² cc-cm/(cm² -sec-cmHg) could be added to reduce the oxygen permeability of a lactic acid residue containing polymer based film.

EXAMPLES Example 1 Example Showing Inverse Relationship Between Tear Resistance and Blocking Resistance and Direct Relationship Between Plasticizer Level and T_(g)

A Leistritz 34 mm twin screw extruder was used to compound a mixture of components described below. The extrudate was cooled in a water bath and chopped into pellets. The pellets were then coated with 0.1% Ultra-Talc 609 to prevent agglomeration, dried at 30° C., and extruded through a flat die to form a structure for property testing.

The twin screw extruder was operated with zone 1 (pellet feed zone) at 150° C., zone 2 at 160° C., zones 3-6 at 170° C., zones 7-8 at 165° C., and zones 9-11 at 160° C. The screw speed was set at 200 rpm. Pellets of polylactide (PLA) polymer, which is a copolymer of lactide with 0.35 wt. percent of epoxidized soybean oil and having a number average molecular weight of 104,000 and a D-level of 11%, available from Cargill, were fed into zone 1 at a rate of 123 g/min using an AccuRate feeder. A plasticizer, acetyl tri-n-butyl citrate from Morflex, Inc., was injected into zone 3 of the extruder at ambient temperatures using a liquid injection system. The plasticizer was fed in at a rate of 31.5 g/min providing a composition of 20.4% plasticizer.

The compounded mixture containing 20% plasticizer was then dry blended with sufficient amounts of the PLA used in the initial compounding (Mn-104,000, D-level of 11%) to obtain mixtures of 0, 5, 10, 15, and 20% plasticizer. These mixtures were then extruded on a 3/4" Killion extruder, through a six inch flat die, into film having a 3.25 mil thickness (0.00325"). The Killion extruder operated with zone 1 at 280° F., zone 2 at 290° F., zone 3 at 300° F., and zone 4, the adaptor, and the die all at 315° F.

The glass transition temperature (T_(g)) for each film was determined using Differential Scanning Calorimetry (DSC) according to procedure known in the art. A typical procedure includes taking a small sample of the film (5-20 mg) and placing it in a sealed capsule. The capsule is loaded in to the DSC and cooled to a temperature well below the expected T_(g), e.g., -100° C. The sample is then heated at a rate between 5° C./min and 20° C./min and the heat input relative to a blank reference cell is recorded. The glass transition temperatures are evaluated, and recorded as the midpoint of the typical sigmoidal curve. The sample is evaluated on the first upheat of the DSC, to avoid any mixing of the sample phases.

The films were then aged and tested for tear propagation resistance and for blocking resistance. The tear propagation resistance test was conducted according to ASTM Method D 1922-89. The blocking resistance test involved placing two films on top of each other and placing thereon a 400 gm weight with a 2.2 in² contact area. This was left in a temperature controlled environment for 2.0 hours at 50° C. and checked for blocking. The blocking scale for this test ranges from 0 for no blocking to 5 for complete fusion of the two layers. The results of the tear propagation resistance and blocking resistance tests are provided in Table 1.

                  TABLE 1                                                          ______________________________________                                                                              Normalized                                                                     Elmendorf                                 Percent                                                                               Elmendorf Tear (gm.sub.f)                                                                    Blocking        (gm.sub.f /mil)                           Plasticizer                                                                           MD (avg) TD (avg) Level   T.sub.g (°C.)                                                                 MD   TD                                 ______________________________________                                         0      18        14      1       60    5.5  4.3                                5      19.7     21.8     2       52    6.1  6.7                                10     31.2     26.33    4       41    9.6  8.1                                15     704      816      5       30    220  250                                20     1,510    1600+    5       19    460  490                                ______________________________________                                    

The results in Table 1 indicate that blocking resistance is inversely related to tear propagation resistance for single layer, plasticized films of polylactide. The results further indicate that the glass transition temperature (T_(g)) is directly related to the amount of plasticizer therein. It is a discovery of the present invention that highly plasticized films of polylactide, while providing the desired properties of low T_(g) and high tear strength, develop severe blocking problems.

The normalized elmendorf tear values are used to get approximate tear values of a 1 mil film.

Example 2 Example Showing Multilayer Film with Good Blocking Resistance and Good Tear Propagation Resistance

A multilayer film was produced on a 10", four layer, Streamlined Coextrusion Die (SCD) blown film die manufactured by Brampton Engineering. Layer configuration of the die is as follows from outside to inside layers of the die, A/B/D/C. Three 31/2" David Standard extruders fed the A, D, and C layers while a 21/2" David Standard extruder fed B layer. The process line also utilized a Brampton Engineering rotating air ring for polymer cooling. Layers B and D contained PLA (Mn=103,000, D-level of 11%) plasticized with 20% Citroflex which was compounded as described in Example 1. Layers A and C contained PLA (Mn=66,000, D-level of 3%) dry blended with 10% Biopol D300G, supplied by Zeneca Corporation. Layer ratios for the film were A-19%, C=21%, combination of B and D=60% of the total film structure. The thickness of the film produced was 2.25 mil (0.00225"). The processing conditions for the film are provided in Table 2.

                  TABLE 2                                                          ______________________________________                                                Extruder A                                                                             Extruder B                                                                               Extruder C                                                                               Extruder D                                  ______________________________________                                         Zone 1   300       300       300     300                                       Zone 2   310       310       310     310                                       Zone 3   320       320       320     320                                       Zone 4   340       340       340     340                                       Zone 5             340       340                                               Scn Chngr                                                                               330       330       330     330                                       Adapter 1                                                                               330       330       330     330                                       Adapter 2                                                                               330       330       330     330                                       Adapter 4                                                                               330       330       330     330                                       Die 1    330       330       330     330                                       Die 2    330       330       330     330                                       Die 3    330       330       330     330                                       Pressure 1,280     1,670     1,640   1,310                                     Melt Temp                                                                               336       338       338     339                                       Screw Spd                                                                               14        50        48      12                                        Amps     50        40        45      120                                       Line Spd 122 fpm                                                               Notes    PLA/Biopol                                                                               Plasticized                                                                              Plasticized                                                                            PLA/Biopol                                         blend     PLA       PLA     blend                                     ______________________________________                                          Note: Temperatures in table 2 are given in °F.                    

Tear propagation resistance and blocking resistance testing was conducted on the multilayer film according to the procedure described in Example 1. The test results are provided in Table 3. Additionally, the multilayer film exhibited no sign of blocking when tested at 70° C. for 24 hours.

                  TABLE 3                                                          ______________________________________                                                                     Normalized                                                                     Elmendorf                                          Elmendorf Tear (gm.sub.f)   Tear (gm.sub.f /mil)                               MD (avg)  TD (avg) Blocking Level                                                                              MD   TD                                        ______________________________________                                         112       242      0 @ 70° C.                                                                           50   107                                       ______________________________________                                    

The results in Table 3 indicate that the non-plasticized outer layers prevent blocking while the plasticized inner layers provide the tear resistance of the film.

The normalized Elmendorf tear, although not recommended by ASTM, can be used to provide an estimate of the tear strength of a 1 mil film.

Example 3 Example Showing Bag Making Ability on a Commercial Line

The multilayer film prepared in Example 2 was converted into bags using an in-line bag machine manufactured by Battenfeld Gloucester Engineering Co., Inc. downstream of the extrusion line nips. This example demonstrates the feasibility of making bags on a commercial scale line, and at commercial speeds, from multilayer films according to the present invention.

Example 4 Example Showing Importance of Viscosity Matching

Film samples were run on a 6" 7-layer SCD blown film die manufactured by Brampton Engineering, Inc. with a die gap of 0.060" and a Uni-Flo air ring for film cooling. Labeling of the die layers are from outside to inside A, B, C, D, E, F, G. Layers B, C, D, E, and F were fed by five 30 mm Brampton extruders. Layers A and G were fed by two 45 mm Brampton extruders. In making the film samples layers A and G were filled with 0.85 MI (melt index) polyethylene and cooled down to 100° F. to "freeze" the layers and effectively make the die a five-layer system.

Layers C, D, and E contained PLA (Mn=102,000) plasticized with 20% Citroflex which was compounded as described in Example 1. Layers B and F contained PLA dry blended with 10% Biopol D300G, supplied by Zeneca Corporation. Two different molecular weight PLA were used in the blends for the B and F layers. This allowed us to determine the effects of viscosity on the ability to minimize the film's outer layer percentage. Mn's of 66,000 and 102,000 were utilized. Rabinowitsch corrected viscosities at 177° C. and 200 s⁻¹ for the core layer material was approximately 300 Pa.s, and for the 66,000 Mn material it was 500 Pa.s, and 700 Pa.s for the 102,000 Mn material.

In running the films the outer film layers were run to the minimum thickness percentage as possible while maintaining consistent flow and good film appearance. Additionally it was found that when the outer layers were run to their minimum the bubble/tube stability would be jeopardized and the bubble would collapse, resulting in a less of film production. With the Mn of the PLA in the outer layers at 66,000, layer ratios of both B and F lines were obtainable at 5-10% of the total film structure each. This led to the plasticized layers constituting 80-90% of the total film structure. When the Mn of the PLA in the outer layers was at the 102,000 level the lowest ratio for B and F layers obtainable was 20% each. This resulted in the plasticized layers constituting 60% of the total film structure. As the outer layer percentages were reduced during this trial, there were initially flow instability lines apparent on the film at ratios of less than 20% for B and F layers. As the percentages approached 18% the bubble collapsed. Applicants believe this was due to holes generated in the film from the flow instability at the layer interface.

Viscosity matching allows production of films with maximum thickness of the central flexible layer, which is believed to give a maximum tear strength. Viscosity matching can be either through compositional control, as in the present example or through use of a multi-temperature die.

Processing conditions for the films are provided in Table 4.

                  TABLE 4                                                          ______________________________________                                         A                                           G                                  line      B line  C line  D line                                                                               E line                                                                               F line                                                                               line                               ______________________________________                                         Zone 1 100    290     290   290   290   290   100                              Zone 2 100    340     320   320   320   340   100                              Zone 3 100    350     330   330   330   350   100                              Adapter                                                                               100    350     340   340   340   360   100                              Die 1  350                                                                     Die 2  350                                                                     Die 3  350                                                                     Die 4  350                                                                     Die 5  350                                                                     Die 6  350                                                                     Material                                                                              0.85   PLA/    Plasti-                                                                              Plasti-                                                                              Plasti-                                                                              PLA/  0.85                                    MI     Biopol  cized cized cized Biopol                                                                               MI                                      PE     Blend   PLA   PLA   PLA   Blend PE                               ______________________________________                                    

Example 5 Example showing Disadvantage of Having no Anti-blocking Agent in the Outer Layers

Films were prepared on the equipment described in Example 4 under the same processing conditions. Plasticized PLA was compounded as in Example 1 with the exception that 0.1 wt. % LUPERCO 233-XL (ethyl 3,3-di-(t-butylperoxy)-butyrate which is a peroxyketal) supplied by Elf Autochem was added to compound. This was then fed into C, D, and E layers. PLA and PLA blended with Biopol D300G were utilized in the B and F layers to determine the effects of adding anti-blocking agents to the outer layers of the films. The PLA used had an Mn of 66,000. The composition containing the Biopol consisted of 95% PLA and 5% Biopol D300G. The Biopol D300G was utilized as an anti-blocking agent in this instance.

During extrusion of the films, the sample containing only PLA in layers B and F showed blocking at a level 3, as described in Example 1, upon collapsing the bubble at the take-off nip. The film containing the blend of PLA and Biopol in the B and F layers showed blocking at a level 0 upon collapsing the bubble. Additional testing of the film for blocking resistance was completed in accordance with the test described in Example 1 except that the films were tested for 24 hours.

                  TABLE 5                                                          ______________________________________                                         Outer Layer Composition                                                                     Nip Blocking Level                                                                           Blocking Level in Test                              ______________________________________                                         PLA          3             5                                                   PLA/Biopol Blend                                                                            0             1                                                   ______________________________________                                    

Careful selection of the outer-layer material is important for optimal blocking resistance.

Example 6 Example showing Use of Non-PLA Materials in Multi-Layer Film Structures

Films were produced using the equipment and processing conditions described in Example 4 to produce structures containing materials other than PLA as the base material for one or more of the film layers. When plasticized PLA was utilized it was prepared in accordance to the method described in Example 1. Again only the B, C, D, E, and F layers of the die were utilized to produce the films. In all of these films there was an attempt to make a flexible core layer surrounded by rigid, non-blocking outer layers.

                  TABLE 6                                                          ______________________________________                                         Material                                                                       Sample #                                                                              Layer B Layer C   Layer D Layer E Layer F                               ______________________________________                                         1      PLA     PVOH      PVOH    PVOH    PLA                                   2      PLA     Polyethylene                                                                             Polyethylene                                                                           Polyethylene                                                                           PLA                                   3      Biopol  Plasticized                                                                              Plasticized                                                                            Plasticized                                                                            Biopol                                               PLA       PLA     PLA                                           4      EVOH    Plasticized                                                                              Plasticized                                                                            Plasticized                                                                            EVOH                                                 PLA       PLA     PLA                                           ______________________________________                                    

In all of the above cases except for sample #3 there was poor adhesion between the PLA layers and the "other" material layers. In the case of sample #3 a film with 5% layer ratios for B and F layers was accomplished. This film showed no blocking at the haul off nip and also demonstrated a blocking level of 0 when tested in accordance to the test described in Example 1. The PVOH was supplied as VINEX 2144 by Air Products And Chemicals, Inc., the PE was a LLDPE (grade 2045) supplied by Dow, and the EVOH was supplied by Eval Corporation.

Example 7 Example Showing Effects of Layer Ratios on Physical Properties

Films were processed on the equipment described in Example 4 to produce films with varying layer ratios. The outer layers, layers B and F, utilized PLA dry blended with 10% Biopol D300G. The inner layers of the film, layers C, D, and E, utilized PLA compounded with 20% Citroflex plasticizer as described in Example 1. The following films were produced with thickness ranging from 1.5-1.75 mil (0.0015"-0.00175").

                  TABLE 7                                                          ______________________________________                                                  B Layer     C,D,E, Combined                                                                            F Layer                                       Sample # Ratio (%)   Layer Ratio (%)                                                                            Ratio (%)                                     ______________________________________                                         1        25          50          25                                            2        20          60          20                                            3        15          70          15                                            4        10          80          10                                            5        5           90          5                                             ______________________________________                                    

The films were conditioned in a 50% relative humidity chamber at 20°-25° C. and tested for tear propagation resistance according to ASTM D-1922, tensile properties according to ASTM D-882 method A3, and impact resistance according to ASTM D-3420 with the results provided in Table 8.

                                      TABLE 8                                      __________________________________________________________________________        Tear           Ultimate Ultimate Yield   Yield   Tensile                    Outer                                                                             Resistance (g.sub.f)                                                                    Normalized                                                                           Tensile (psi)                                                                           Elong. (%)                                                                              Tensile (psi)                                                                          Elong. (%)                                                                             Modulus                                                                              Impact               Layer                                                                             (Avg)                                                                              (Avg)                                                                               Tear  (Avg)                                                                              (Avg)                                                                               (Avg)                                                                              (Avg)                                                                               (Avg)                                                                              (Avg)                                                                              (Avg)                                                                              (Avg)                                                                              (kps) Strength             (%)                                                                               MD  TD   (gm.sub.f /mil)                                                                      MD  TD   MD  TD   MD  TD  MD  TD  MD TD (mJoule)             __________________________________________________________________________     25 36.5                                                                               47.5 23 29 5,474                                                                              4,878                                                                               296 342  4,536                                                                              3,738                                                                              5.4 4.6 84 81 231                  20 40.5                                                                               46.5 25 28 5,670                                                                              4,813                                                                               329 317  3,838                                                                              3,418                                                                              5.2 6.2 74 55 643                  15 49  64.5 31 40 5,189                                                                              4,476                                                                               351 313  3,544                                                                              2,673                                                                              5.6 6.1 63 44 660                  10 56.5                                                                               89.5 35 56 5,053                                                                              4,299                                                                               340 343  2,470                                                                              2,087                                                                              5.4 4.5 46 46 812                  5  94.3                                                                               128  58 80 4,921                                                                              4,845                                                                               367 422  1,916                                                                              1,608                                                                              6.2 9.8 31 16 1,236                __________________________________________________________________________

The data in Table 8 demonstrates the effect the layer ratios have on the tear propagation resistance, the yield strength, and the impact strength. Increasing the thickness of the inner, flexible layer, and decreasing the thickness of the outer layers, provides high tear resistance and impact strength, although yield is reduced. The data also demonstrates that the layer ratios do not have much of an effect on the ultimate tensile strength, the ultimate elongation, or the yield elongation.

The normalized tear force is provided as an estimate for a 1 mil film, although it is preferred to test a 1 mil film directly.

Example 8 Example Showing non-Migratory Plasticizer in Coextruded Film

The film produced in Example 2 was run on a DSC to determine the T_(g) of the two distinct types of PLA present in the structure. Evaluating the shift in T_(g) over time provides an indication of plasticizer migration or lack of migration. An upward shift in the core (plasticized) layer's T_(g) would indicate migration of plasticizer out of the core layer. This would correspond to a downward shift of the outer layer T_(g) as the plasticizer migrated into these layers. The DSC test results are provided in Table 9.

                  TABLE 9                                                          ______________________________________                                         DSC Test Date                                                                            T.sub.g1 (°C.) - for core layer                                                        T.sub.g2 (°C.) - for outer                     ______________________________________                                                                  layer                                                 12/27/94  18.3           53.45                                                 01/04/95  18.98          54.07                                                 01/20/95  17.49          53.05                                                 02/03/95  15.24          56.30                                                 08/17/95  16.76          53.77                                                 ______________________________________                                    

The data in Table 9 demonstrates that there is no appreciable change in the T_(g) of the layers of the film structure over an 8 month period. This indicates a lack of migration of the plasticizer over time for the tested multilayer film structure.

Example 9 Example Showing Compostable Properties of Coextruded Film

The multilayer film of Example 2 was tested to determine the rate and extent of degradation in a compost environment. For comparative purposes, sample of kraft paper and cellulose were similarly tested to evaluate the rate and extent of degradation in a compost environment. The kraft paper was from a typical grocery bag and the cellulose was microcrystalline cellulose from Avicel.

For all three samples, a modified version of ASTM D5338-92 test was performed according to ASTM D5338-92 except that a constant temperature of 58° C. was provided in order to more conveniently approximate natural composting conditions. The amount of material biodegraded was calculated based upon measuring the amount of carbon dioxide evolved therefrom.

Test results were plotted in the graph of FIG.3 as "Cummulative % Biodegradation (CO2-C)" as a function of time. The graph demonstrates that the multilayer film of Example 2 degrades at a rate and to an extent fairly close to cellulose and kraft paper.

While the invention has been described in conjunction with specific embodiments thereof, it is evident that different alternatives, modifications, variations, and uses will be apparent to those skilled in the art in view of the foregoing description. Accordingly, the invention is not limited to the embodiments presented herein. 

What is claimed:
 1. A compostable multilayer structure comprising:(a) core layer having first and second opposed surfaces, said core layer includes lactic acid residue-containing polymer containing at least about 50%, by weight, lactic acid residue units, and an effective amount of plasticizer to provide the core layer with a T_(g) below about 20° C.; (b) first blocking reducing layer covering the first surface of said core layer, said first blocking reducing layer having a T_(g) above about 50° C. and comprising a hydrolyzable polymer; and (c) second blocking reducing layer covering the second surface of said core layer, said second blocking reducing layer having a T_(g) above about 50° C. and comprising a hydrolyzable polymer,wherein said blocking reduced layers provide reduced blocking compared with said core layer.
 2. The compostable multilayer structure according to claim 1, wherein said lactic acid residue-containing polymer is a polylactide polymer.
 3. The compostable multilayer structure according to claim 2, wherein the polylactide polymer is a copolymer prepared by reacting lactide monomer with non-lactide or non-lactic acid monomer.
 4. The compostable multilayer structure according to claim 3, wherein the non-lactide monomer is epoxidized multifunctional oil.
 5. The compostable multilayer structure according to claim 2, wherein at least one of said blocking reducing layers comprises an amorphous polylactide polymer, a semicrystalline polylactide polymer, or a mixture thereof.
 6. The compostable multilayer structure according to claim 1, wherein said compostable multilayer structure has a biodegradability value of about 20 percent or higher after 40 days according to ASTM D5338-92, modified to test at about 58° C.
 7. The compostable multilayer structure according to claim 1, wherein said compostable multilayer structure has a biodegradability value of about 50 percent or higher after 40 days according to ASTM D5338-92, modified to test at about 58° C.
 8. The compostable multilayer structure according to claim 1 having a tear resistance greater than 25 gm_(f) /mil.
 9. The compostable multilayer structure according to claim 1, wherein said core layer has a T_(g) less than about 5° C.
 10. The compostable multilayer structure according to claim 1, wherein said core layer is compounded with a peroxyketal to reduce plasticizer migration.
 11. The compostable multilayer structure according to claim 1, wherein said core layer comprises at least two sublayers, wherein each sublayer comprises a hydrolyzable polymer.
 12. The compostable multilayer structure according to claim 11, wherein said sublayers have different properties of flexibility and tear resistance.
 13. The compostable multilayer structure according to claim 1, wherein said core layer comprises at least two sublayers, wherein at least one sublayer has a T_(g) below about 20° C. and a second sublayer has a water vapor transmission rate of less than about 5 g-mil/100 in² -day.
 14. The compostable multilayer structure according to claim 1, wherein said structure is transparent to visible light.
 15. The compostable multilayer structure according to claim 1, wherein said core layer comprises a hydrolyzable polymer having a number average molecular weight greater than about 80,000.
 16. The compostable multilayer structure according to claim 1, wherein said first blocking reducing layer and said second blocking reducing layer comprise a hydrolyzable polymer having a number average molecular weight greater than about 50,000.
 17. The compostable multilayer structure according to claim 1, wherein at least one of said blocking reducing layers comprises an anti-blocking agent.
 18. The compostable multilayer structure according to claim 17, wherein the anti-blocking agent is a polymer additive having a glass transition temperature above about 50° C. or a semicrystalline polymer additive having a melting point above about 90° C.
 19. The compostable multilayer structure according to claim 18, wherein the polymer additive is selected from the group consisting of cellulose acetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate, cellulose acetate butyrate, and cellulose propionate butyrate.
 20. The compostable multilayer structure according to claim 18, wherein the semicrystalline polymer additive is selected from the group consisting of poly(hydroxy butyrate), poly(hydroxy butyrate -co- hydroxy valerate), poly(glycolide), poly(lactide), stereocomplex of poly(L-lactide) and poly(D-lactide), and mixtures thereon.
 21. A compostable multilayer sheet comprising:a multilayer structure prepared according to claim
 1. 22. The compostable multilayer structure according to claim 1, wherein said core layer is at least about 70% of the thickness of the multilayer structure.
 23. The compostable multilayer structure according to claim 1, wherein the first blocking reducing layer has a thickness of between about 0.05 mil and 0.5 mil.
 24. A compostable multilayer structure according to claim 1, wherein the core layer comprises a polymer blend of poly(lactide) polymer and at least one different hydrolyzable polymer selected from the group consisting of poly(glycolide), poly(hydroxybutyrate), poly(hydroxybutyrate-co-hydroxyvalerate), poly(caprolactone), poly(ethylene-oxylate), poly(1,5-dioxepan-2-one), poly(1,4-dioxepan-2-one), poly-(p-dioxanone), poly(delta-valerolactone), poly(trimethylene carbonate), and polyester.
 25. A compostable multilayer structure according to claim 1, wherein the core layer comprises a polymer blend of poly(lactide) polymer and at least one different hydrolyzable polymer comprising aliphatic polyester.
 26. A compostable multilayer structure comprising:(a) a compostable substrate; (b) a core layer having first and second opposed surfaces, said core layer includes lactic acid residue-containing polymer containing at least about 50%, by weight, lactic acid residue units, and an effective amount of plasticizer to provide the core layer with a T_(g) below about 20° C.; and (c) block reducing layer covering said core layer, said blocking reducing layer having a T_(g) above about 50° C. and comprising hydrolyzable polymer, wherein said blocking reducing layer provides reduced blocking compared with said core layer.
 27. The compostable multilayer structure according to claim 26, wherein said compostable substrate is a cellulose-containing substrate.
 28. The compostable multilayer structure according to claim 27, wherein the cellulose-containing substrate is paper.
 29. A compostable bag comprising:a multilayer structure prepared according to claim
 26. 